Diamond wafer, method of estimating a diamond wafer and diamond surface acoustic wave device

ABSTRACT

Surfaces of diamond crystals are examined by coating the surfaces with thin metal films, launching laser beams to the diamond surfaces in a slanting angle, detecting defects and particles on the diamond surfaces by the scattering of beams and counting the defects and particles by a laser scanning surface defect detection apparatus. Diamond SAW devices should be made on the diamond films or bulks with the defect density less than 300 particles cm −2 . Preferably, the diamond surfaces should have roughness less than Ra20 nm. Diamond SAW filters can be produced by depositing a piezoelectric film and making interdigital transducers on the low-defect density diamond crystals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a diamond wafer and a surfaceacoustic wave (SAW) device produced on the diamond wafer. This inventionrelates more specially to a method of estimating diamond wafers whichjudges whether a diamond wafer is suitable for producing SAW devices ornot. The estimation selects diamond wafers that have a low density ofsurface defects which enables manufacturers to produce a low loss SAWdevice. The diamond wafers can also be used as the substrates ofmicroelectronic devices with microscopic wiring or the substrates ofmicromachines with microscopic structures.

This application claims the priority of Japanese Patent ApplicationNo.9-290262 (290262/97) filed Oct. 6, 1997 which is incorporated hereinby reference.

2. Description of Related Art

Diamond enjoys the highest sound velocity among all the naturalmaterials. The hardness is also the highest. The thermal conductivity islarge. The band gap of diamond is 5.5 eV which is an extremely highvalue among all known materials. Diamond is excellent in dynamicalproperty, electrical property and electronic property. Diamond is usedfor dynamic devices and electronic devices that can take advantage ofits excellent properties. Improvements have been made by takingadvantage of the outstanding properties of diamond in the technicalfields of acoustics, optics and semiconductor. Exploitation of diamondwill be effective for improving various properties of electronic,acoustic devices or for enlarging margins of operation of the devices.

A surface acoustic wave device (SAW device) is a good candidate for theuse of diamond for improving its characteristics. Surface acoustic wavedevices can be configured to be, for example, a radio frequency filter,a phase shifter, a convolver, an amplifier, etc. The SAW filter acts asan IF (intermediate frequency) filter of television sets or variousfilters of communication devices. A surface acoustic wave device is adevice having a rigid base, a piezoelectric film stuck to the rigid baseand interdigital transducers formed on both ends of the piezoelectricfilm. Application of an AC voltage on the interdigital transducer causesan AC electric field on the piezoelectric film which deforms inproportion to the electric field. Since the electric field oscillates,the piezoelectric film alternatively expands and contracts in thehorizontal direction between two interdigital transducers at the samefrequency as the AC voltage.

The piezoelectric film oscillates in the horizontal direction with thefrequency of the AC signal. Since the piezoelectric film adheres to therigid base, the rigid base also oscillates at the same frequency in thesame manner. Since the rigid base repeats expansion and contraction atthe interdigital transducer (IDT), the oscillation propagates as alongitudinal elastic wave on the surface. The AC voltage applied on theinterdigital transducer generates an elastic wave. The wavelength isdetermined by the period of the interdigital transducer. The elasticwave spreads from one interdigital transducer to the other interdigitaltransducer. The piezoelectricity is reversible. At the receivinginterdigital transducer, the deformation oscillation induces an ACvoltage between the components of the electrode. As a whole, the ACsignal propagates from one interdigital transducer to the otherinterdigital transducer by the elastic wave. The wave is called surfaceacoustic wave (SAW), because it propagates on the surface of the device.

The period of the interdigital transducer uniquely determines thewavelength λ of the surface acoustic wave. The rigidity and density ofthe rigid base determine the velocity v of the SAW. The more rigid andlighter base brings about the higher SAW velocity. SAW velocity v isdifferent from sound velocity which is equal to a square root of Young'smodulus divided by density ρ. As with sound velocity, the SAW velocityis higher for the rigid base of higher Young's modulus and lowerdensity. A sound wave is an elastic wave passing through an innerportion of a material. SAW is another elastic wave propagating only onthe surface of the material. SAW differs from sound wave. SAW velocityis, in general, higher than sound velocity.

Since the wavelength λ and the velocity v have been predetermined by theinterdigital transducer and the physical property of the rigid base, thefrequency f is also surely determined as f=v/λ. This is a unique value.Since f is a unique value, it is denoted by f₀. Namely, the SAW devicehas a filtering function which selectively allows only the SAW of f₀ topass the device. SAWs of frequency different from f₀ attenuate.Transmittable SAW has a definite frequency f₀ which is determined by thematerial of the rigid base and the spatial period of the interdigitaltransducers. SAW devices have been applied to TV filters having a lowallowable frequency of several megahertzs to tens of megahertzs.Hopefully, SAW devices will be applied to far higher frequency filters,for instance, optical communication filters of 2.488 GHz or wireless LANfilters in near future.

Raising frequency f₀ requires either narrowing a spatial period ofinterdigital transducers or increasing a SAW velocity v. The spatialperiod of interdigital transducers is limited by the current lithographytechnology. The only way is the increase of velocity v Diamond, as arigid base, exhibits the highest SAW velocity among all naturalmaterials. The application of diamond to SAW devices attracts attention.Diamond endows the SAW devices with the highest velocity which affords amoderately wide spatial period to the interdigital transducers.

High velocity is not the only requisites for a material used for SAWdevices. Low propagation loss is another important requirement for SAWmaterials. Loss is a key concept of the present invention. There aredifferent losses in addition to the propagation loss. Losses are nowbriefly clarified. One is the Joule loss ΔEr by the resistance of theinterdigital transducers to which electric power is supplied. Another isan electromechanical conversion loss ΔEc of energy accompanying theexpansion and contraction of the piezoelectric material by the ACelectric field. The loss depends on the electromechanical coefficient ofthe piezoelectric film. The interdigital transducer which convertselectric power into mechanical power through the piezoelectric film hasno selectivity of direction. The surface acoustic waves propagate inboth directions perpendicular to the stripes of the interdigitaltransducers. Just half of the mechanical power spreads toward thecounterpart interdigital transducer. Another half (6 dB) is a loss. Thisis called a bisection loss ΔEb. Now a SAW starts from one interdigitaltransducer and some of the SAW arrives at the other interdigitaltransducer. The difference between the starting SAW power and arrivingSAW power is the propagation loss ΔEp. The aim of the present inventionis a reduction of the propagation loss ΔEp. At the other interdigitaltransducer, the piezoelectric film converts the mechanical power of SAWinto electric power of AC voltage with a conversion loss ΔEc. Thecurrent flows in the receiving interdigital transducer with a resistanceloss ΔEr.

Total loss is a sum, 2 ΔEr+2 ΔEc+ΔEb+ΔEp, of the resistance loss 2 ΔEr,the conversion loss 2 ΔEc, the bisection loss ΔEb and the propagationloss ΔEp. ΔEr is contingent upon interdigital transducers. ΔEc is ruledby the piezoelectric material. Geometry decides ΔEb. Only thepropagation loss ΔEp depends upon the insulating material (rigid base).This invention aims at alleviating ΔEp.

The insulator which has been most widely used as the material of therigid base is glass. Glass is an inexpensive and low-loss insulator.ZnO/glass SAW filters have been popularly employed as TV intermediatefrequency filters. Zinc oxide (ZnO) is a piezoelectric material.ZnO/glass means a SAW filter having a glass substrate and a ZnO filmdeposited on the glass. In spite of low-loss and low-cost, glass SAWdevices cannot raise operation frequency f₀ owing to the low SAWvelocity v. Somebody has proposed new SAW devices having a harder rigidbase than glass, for example, sapphire, quartz, LiNbO₃ and so on. Thesenew materials give higher SAW velocity than glass owing to high Young'smodulus. However, the SAW velocities of sapphire SAW devices, quartz SAWdevices or LiNbO₃ SAW devices are still unsatisfactory for highfrequency filters. Diamond is the most promising candidate which givesthe highest SAW velocity due to the extreme rigidity.

Diamond SAW devices enable the current lithography technology to patterninterdigital transducers for a frequency higher than 1 GHz owing to thehigh SAW velocity v. However, the propagation loss ΔEp is still large indiamond SAW devices. The large propagation loss ΔEp leaves diamond SAWdevices impractical. It is difficult to make a wide, flat, even, smoothand defect-free diamond film covering the whole of the device due to theextreme rigidity of diamond. It is further difficult to make a flatpiezoelectric film on the defect-rich diamond film. Even if thepiezoelectric film is produced, it is still a piezoelectric film of poorquality. Many defects on the diamond surface and the shortness of theSAW wavelength raise the propagation loss ΔEp and leave diamond SAWdevices inoperative.

Since the frequency f is very high, the wavelength λ=v/f is reduced to asimilar size to the micro-cavities and micro-convexities. Acousticphonons building the surface acoustics wave are scattered by themicro-defects, because phonons are mostly perturbed by the objects ofthe same size as the wavelength. Besides the high propagation loss ΔEp,the defects-rich diamond film decreases the yield of SAW devices byraising the rate of electrode-pattern cutting. Micro-defects preventdiamond SAW devices from growing to practical SAW devices through thelarge propagation loss ΔEp and the low yield. Here “yield” means a ratioof the number of good products to the number of all products.

One purpose of the present invention is to produce diamond SAW deviceshaving low propagation loss. Another purpose of the present invention isto enhance the yield of the diamond SAW devices. In addition to SAWdevices, this invention can be applied to raising the yield of theproduction of microelectronic devices or micromachines making use ofsmall wire patterns with a breadth less than 5.0 μm through reducing therate of the wiring pattern cutting.

Higher frequency requires SAW filters to make a still betterpiezoelectric film. The undercoating diamond film must be flat, smoothand defect-free for producing (Diamond)/(piezoelectric material) SAWdevices. For this purpose, a simple and reliable estimation technique ofdiamond films is indispensable besides fabrication of good diamondfilms. This invention proposes also a method for easily estimatingdiamond films.

Diamond films have been examined by observing micro-defects,micro-concavities or micro-convexities by scanning electron microscopesor atomic force microscopes.

{circle around (1)} R. Gahlin, A. Alahelisten, S. Jacobson, “The effectsof compressive stresses on the abrasion of diamond coatings”, Wear 196(1996) 226-233

{circle around (2)} S. K. Choi, D. Y. Jung, S. Y. Kweon, S. K. Jung,“Surface characterization of diamond films polished by thermomechanicalpolishing method”, Thin Solid Films 279 (1996) 110-114

Microscope estimation is a straight and reliable method, since diamondsurface is directly observed by a microscope. However, the field ofvision is too narrow. The observation field is, e.g., about 10 μm×10 μmfor a scanning electron microscope. In general, a SAW device has a sizeof about 100 μm×100 μm to 20000 μm×20000 μm. The area of a SAW device isfar broader than the field of vision of microscopes. It would take verymuch time to observe the whole surface of a SAW device by microscopes.Microscope observation cannot be applied to the examination of diamondsurfaces in the process of producing diamond SAW devices. Namely, thereis no estimation method available for examining diamond surface on anindustrial scale.

Producing diamond SAW devices requires a comprehensive estimation methodof examining the entier diamond surface at a stretch. If the diamondfilm can be easily examined, one can judge whether a piezoelectric filmshould be deposited on it or not. If the diamond film is bad, the sampleshould be abandoned without coating with a piezoelectric film. Anotherpurpose of the present invention is to propose a method of estimatingphysical property (roughness, defects) of a diamond film.

The Inventors of the present invention tried to utilize a laser scanningsurface defect detection apparatus for silicon wafers to examine diamondfilms of SAW devices. Diamond is different from silicon in many physicalproperties. A laser scanning surface defect detection apparatus forsilicon wafers cannot be readily used for estimating a diamond. Indeed,a laser scanning surface defect detection apparatus has never been usedto examine diamond films. FIG. 4 shows a schematic view of the laserscanning surface defect detection apparatus.

The apparatus has an integral sphere, a laser, a lens and aphotomultiplier. The inner wall of the integral sphere is a mirror. Theintegral sphere has an opening at the bottom. The semiconductor laser(L) is mounted at a niche on a side of the integral sphere for emittingan inspecting light beam slantingly down. The semiconductor laser can bereplaced by a gas laser, e.g. a He-Ne laser. The integral sphere has anoutlet (U) on the other side for taking out a reflected beam. Theinspection comprises the steps of mounting an object wafer on a stage,bringing the bottom opening of the integral sphere close to the wafer(KTW), irradiating a point (T) on the surface of the wafer by the laserbeam (LT), and measuring the power of the reflected light (S). Theincident beam angle is equal to the refection angle. The laser (L) andthe outlet (U) are determined to be symmetric in the integral sphere.Then, ∠LTK=∠UTW. If the surface is flat, all the reflected beams go outthrough the side opening (U). The photomultiplier detects no light. Whena defect or a piece of dust lies on the object point (T), a portion ofthe laser beams is scattered by the defect or dust. The scattering beamsimpinge upon other parts of the wall of the integral sphere. Thephotomultiplier senses the scattered beams. FIG. 13 shows a bit of dustlying on the surface of the wafer. Parallel laser beams are randomlyreflected by the dust in the directions E, F, G, etc. Reflected beams donot exit through the outlet (U) but impinge upon the inner wall of theintegrated sphere and are reflected to the photomultiplier. Thephotomultiplier measures the scattering light with high sensitivity. Thephotomultiplier detects the existence of a dust at the sampling point(T) by an increase of its photocurrent. The bigger the dust is, thestronger the scattering light becomes. The reflected light power is inproportion to the size of the dust. It is easy to understand thescattering of light by dust. However, the photomultiplier can alsodetect geometrical surface defects. FIG. 14 shows a notch, a surfacedefect, on the surface of the silicon wafer. Silicon has a highrefractive index (n=3.5) and high absorption for the visible lightemitted from the laser. The complex refractive index n+jk of silicon hasa large real part n and a large imaginary part k for the visible light.Laser light is strongly reflected at the interface between air andsilicon. The notch (M scatters the beam in other directions (MH) thanthat of the outlet (U). The scattered beams are again reflected by theinner wall of the integral sphere and arrive at the photomultiplier forgenerating a photocurrent. The existence of dust or a defect on theobject point of the silicon wafer can easily be detected by the randomscattering of light beams.

Since a Si wafer is a wide disc, defects and dust of all the portionsare inspected by moving the stage in the two-dimensional XY-plane withregard to the integral sphere. There are two modes of movement of thestage. One is an assembly of rotation and parallel displacement of theshaft. The sampling point (T) moves in a spiral. This mode is called aspiral mode. The other is an assembly of x-direction displacement andy-direction displacement of the wafer. This is called a scanning mode.The overall movement enables the apparatus to detect dust or defects onthe whole surface of the silicon wafer.

In the silicon semiconductor industry it is well known to use a laserscanning surface defect detection apparatus. It takes only a short timeto examine a pretty wide silicon wafer by the laser scanning surfacedefect detection apparatus. Unlike observation by a microscope, theapparatus can measure the number or the density of defects or dustquantitatively over a wide range. However, the laser scanning surfacedefect detection apparatus cannot be readily used to examine the surfaceof diamond.

Diamond is transparent for the visible light emitted by the laser unlikesilicon. Namely, the complex refractive index n+jk has a vanishingimaginary part k=0. The real part n=2.4 is also lower than the real partof silicon. The reflection rate is r=(n−1)/(n+1) for the verticalincidence. Since n is small, the reflection rate is also small.Transparency allows the visible light to penetrate into the diamondcrystal. Any notch (M) on diamond cannot reflect the laser light in FIG.3(a). A visible light beam passes a diamond bulk crystal without lossalong a light path LMP in FIG. 3(a). If a diamond film is formed on anopaque substrate of a foreign material, the light passes the transparentdiamond and arrives at the interface between the diamond film and thesubstrate. The beam is reflected by the foreign material substrate alongthe light path LNQ in FIG. 3(a). Since diamond is transparent, thereflection occurs not on the diamond surface but on the interfacebetween the diamond film and the substrate. For this reason, the laserscanning surface defect detection apparatus cannot be applied to theexamination of the surface of diamond. The Inventors thought that thelaser scanning surface defect detection apparatus is a promisingcandidate for an inspection apparatus for diamond despite the difficultyof transparency.

SUMMARY OF THE INVENTION

One purpose of the present invention is to provide an improvement of thelaser scanning surface defect detection apparatus so that it would besuitable for examining surface defects of diamond. Another purpose ofthe present invention is to provide a diamond substrate which issuitable for use as the substrate of SAW devices by examining surfacedefects by the laser scanning surface defect detection apparatus. Thefurther purpose of the present invention is to provide a method ofselecting diamond substrates suitable to form a piezoelectric filmthereupon.

This invention coats a diamond film or substrate with a thin metal ornon-metal film having a high reflection rate for the laser light. Thethin film enables the laser scanning surface defect detection apparatusto detect defects or dust on a surface of the diamond, since the thinfilm reflects the laser light. The thin film should be made from a metalor non-metal having a reflectance greater than 50%. The thickness of thecoating film is less than 100 nm. The surface of the coating has thesame convexities and concavities as the surface of the diamond, sincethe coating film is thin enough. The coating layer transcribes thediamond surface. If the diamond has a surface defect (M), the coatinglayer has a similar surface defect (K), as shown in FIG. 3(b). Most ofthe beams are reflected at the coating layer. The rest penetrating intothe coating layer is also reflected at the interface between the coatinglayer and the diamond. The material of the coating layer is, e.g.,aluminum (Al), gold (Au), silver (Ag), copper (Cu), nickel (Ni) or so.Thin film of the metal has a sufficient reflection rate to the laserlight. If the diamond has defects or dust, the coating layer has similaranomalies due to the defects or dust at the same spots. The formalanomalies reflect the laser beams at random. Then the laser beams arescattered indirectly by the defects or dust on the diamond. The coatinglayer enables a conventional laser scanning surface defect detectionapparatus to detect defects or dust on an object diamond. Thus, thelaser scanning surface defect detection apparatus can now be used forthe estimation of diamond surfaces.

FIG. 2 shows a diamond crystal covered by a metal coating layer. Thecoated diamond is examined with regard to defects or dust by the laserscanning surface defect detection apparatus. If dust lies on thesurface, the dust itself scatters the laser light. The scattered lightarrives at the photomultiplier. The existence of the dust can be surelydetected by the apparatus. However, defect detection is a problem. Asshown in FIG. 3(b), the metal coating layer has the same geometricanomaly as the defect, because the metal layer is thin enough. Most ofthe laser light is reflected at K point. The scattering beam goes alongKR. A part of the rest is reflected at M point. The scattered beams KRand MI are again reflected at some portions of the integral sphere andreach the photomultiplier. Thus, the existence of the defect is also bedetected by the apparatus. Therefore, the laser scanning surface defectdetection apparatus which is inherently directed to examining Si waferscan now be diverted to the examination of defects and dust on diamondsurfaces by coating the object diamond with a thin opaque layer.

Diamond films are now examined by the laser scanning surface defectdetection apparatus. The Inventors discovered that the critical densityis 300 particles/cm² of the surface defect for diamond films by theexaminations. Diamond films which have more than 300 particles/cm²defects shall be rejected for the substrate of SAW devices. The diamondfilms with defect density less than 300 particles/cm² shall be adoptedas a substrate for SAW devices. The most important factor is the surfacedefect density for estimating diamond films as the substrate of SAWdevices.

The surface roughness Ra of diamond films has an influence upon thequality of the SAW devices in addition to surface defects. The surfaceshould have a surface roughness lower than Ra 20 nm. In particular, lessthan Ra 10 nm is preferable for the substrates of SAW devices. Theroughness is the second criterion for estimating the quality of diamondfilms.

Another factor is the size of a diamond-coating wafer. Raisingproductivity requires a wafer size of more than 2 inches of diameter.Namely, the surface area should be wider than 19 cm² in the case of acircular wafer.

Inner stress is a further factor for defining a desirable diamond-coatedwafer. If a wafer has a big inner stress, the mother materials, siliconor a metal, will be distorted. Distortion of the mother substratedeforms or bends the diamond film either in a convex or in a concave.The deformation of the diamond film reflects the laser light unevenly. Aslight slanting of the wafer forbids the reflected light from going outthrough the outlet U and forces a part of the reflected light into thephotomultiplier. Thus, inner stress should be weaker than 1.5×10⁸ Pa(150 MPa=1500 bar). Besides, the distortion which is defined as theheight of the center from the surface containing the periphery should besmaller than ±40 μm.

Of course, a pure diamond film is most suitable. But a diamond coatedwith a thin carbon is also allowable, since the diamond is directed fora substrate of SAW devices. In the case, the carbon overcoat layershould be less than 10 nm in thickness. FIG. 1 shows the followingrequirements for the diamond film for SAW devices in the presentinvention.

(1) defect density≦300/cm²

(2) surface roughness≦Ra 10 nm

(3) area≧19 cm²

(4) inner stress≦150 MPa

(5) carbon overcoating≦10 nm.

Instead of a film, a freestanding, homogeneous diamond substrate withoutforeign material substrate is also available. In the full diamondsubstrate, the thickness of the diamond crystal should be 100 μm to 2000μm. The complete diamond substrate makes excellent SAW devices.

Complex substrates having a foreign material, e.g. silicon, substrateand a diamond film coating the substrate are, in general, made use forthe substrate of SAW devices. The mother foreign material should have athickness between 0 μm and 2000 μm. At the limit of 0 μm, the casecoincides with the former full homogenous diamond substrate. Thethickness of the diamond film should be more than 1 μm. The mothersubstrate and the diamond film should have a roughness less than Ra 20nm. In particular, the surface roughness of the diamond film preferablyis less than Ra 10 nm.

(6) in the case of a homogeneous full diamond substrate, the thicknessshould be 100 μm to 2000 μm.

(7) in the case of a diamond/mother complex substrate, the mothersubstrate thickness should be 0 μm to 2000 μm.

(8) in the case of a diamond/mother complex substrate, the diamond filmthickness should be more than 1 μm.

(9) the surface roughness of the diamond film or substrate should beless than Ra 20 nm.

Although many proposals have been discussed and proved that diamond SAWdevices would be superior in the SAW velocity to other materials,diamond SAW devices are suffering from extremely low yield due to thelarge propagation loss of the SAW devices having piezoelectric filmsupon diamonds. The low yield resulted from the lack of the method of apreliminary examination of diamond crystals. Since the property ofdiamond films could not be measured, SAW devices made on theproperty-unknown diamond crystals were tested, which lowered the yieldand raised the production cost. This invention provides an estimationmethod which can examine diamond films or bulks at a preparatory step ofmaking diamond SAW devices. This invention makes the best use of thelaser scanning surface defect detection apparatus which prevails insilicon semiconductor industries. The laser scanning surface defectdetection apparatus cannot be employed for examining diamond surfaces asit is. Preparatory coating of thin films of a metal or another opaquematerial enables the laser scanning surface defect detection apparatusto examine the defects of diamond surfaces. The examination reveals thedensity of defects and dust on diamond films or bulks for determiningwhether piezoelectric films and electrodes should be made upon thediamond samples at a preliminary step. Diamond crystals having defect &dust density of more than 300 particles cm⁻² cannot form goodpiezoelectric films and good interdigital transducers thereupon. Higherdefect density than 300 particles cm⁻² lowers the yield by invitingelectrode breaking or other default. Diamond surfaces of a defectdensity lower than 300 particles cm⁻² pass the examination. The passeddiamond enables manufactures to produce low loss SAW filters availablefor high frequency by depositing piezoelectric films and makinginterdigital transducers on the diamond surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic front view of a diamond crystal having theconditions suitable for a substrate of SAW devices.

FIG. 2 is a section of a sample of a diamond crystal coated with a metalfilm available for an examination by a laser scanning surface defectdetection apparatus.

FIG. 3(a) is a view of a defect on a diamond surface without coating,where a laser beam penetrates a transparent diamond crystal.

FIG. 3(b) is a view of a defect on a diamond surface coated with a metalfilm, where a laser beam can detect the defect by being reflected at themetal surface.

FIG. 4 is a sectional view of a laser scanning surface defect detectionapparatus which has been used for examining surfaces of silicon wafers.

FIG. 5 is a sectional view of a Si substrate and a diamond filmdeposited on the Si substrate by a microwave plasma CVD (chemical vapordeposition) method.

FIG. 6 is a sectional view of a Si substrate and a diamond film polishedby a diamond electrodeposited whetstone.

FIG. 7 is a sectional view of a sample having a piezoelectric ZnO filmdeposited on the diamond/Si substrate.

FIG. 8 is a sectional view of a sample having an aluminum film coatingon the ZnO/diamond/Si substrate.

FIG. 9 is a sectional view of a sample having interdigital transducersmade by selective etching of the aluminum film by lithography.

FIG. 10 is a graph of the frequency dependence of S12 of an embodimentSAW filter of the present invention. The abscissa is the frequency (MHz)and the ordinate is the electric power ratio (dB).

FIG. 11 is a graph of the frequency dependence of S11 of the same SAWfilter. The abscissa is the frequency (MHz) and the ordinate is theelectric power ratio (dB).

FIG. 12 is a graph of the frequency dependence of S22 of the same SAWfilter. The abscissa is the frequency (MHz) and the ordinate is theelectric power ratio (dB).

FIG. 13 is a section of a particle of dust lying on the diamond surfacewhich scatters laser beams in random directions.

FIG. 14 is a section of a defect which scatters a laser beam.

FIG. 15 is a plan view of a part of an example of an interdigitaltransducer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Diamond can be synthesized by two methods. One way produces a diamondbulk crystal from carbon at a high temperature under ultrahigh pressure.This bulk-production method is called an ultrahigh pressure synthesis.The other way produces a diamond thin film on a mother substrate bysupplying hydrogen gas, hydrocarbon gas to the mother substrate,exciting the gas by heat, light or microwave, inducing a chemicalreaction of the gas and depositing the reaction product on the mothersubstrate at a high temperature under reduced pressure. Thisfilm-production method is called a vapor phase synthesis. Both methodscan produce both diamond single crystals and diamond polycrystals. Thisinvention can be applied both to single crystals and polycrystals ofdiamond.

However, the ultrahigh pressure method can make only a small-sizeddiamond crystal. A big diamond wafer of an area wider than 19 cm² (2inches in diameter) can be produced only by the vapor phase synthesis.In any cases, synthesized diamond crystals have some surface roughnesswhich forbids making a uniform piezoelectric film on the diamond. Then,the diamond film should be polished mechanically or chemically.

This invention makes a diamond crystal shown in FIG. 2 by synthesizing adiamond crystal from materials by the methods, polishing the synthesizeddiamond crystal and coating the polished diamond with a thin metal filmof a thickness less than 100 nm. The diamond crystal is then examined bythe laser scanning defect detection apparatus shown in FIG. 4. The metalthin film reflects the laser beams at point (T) to the outlet (U), if nodefect exists at point (T). Hereafter the word “defects” broadly includeintrinsic, crystallographical defects, hillocks, protrusions,concavities and dust particles on a crystal surface in the description.Geometric defects of the diamond are transcribed to the metal surface asshown in FIG. 3(b). If a defect (a dust particle or an intrinsic defect)lies on the interface between the metal and the diamond, the laser beamsare scattered to some spots of the integration sphere and are reflectedagain to the photomultiplier. The photomultiplier senses the defects(dust particles or inherent defects) at point (T) by an increase of thephotocurrent. Scanning the integration sphere on the wafer reveals thetwo-dimensional distribution of the defects including inherent defectsand particles. 300 particles/cm² is the critical value for defectdensity. Diamond wafers having a defect density of more than 300particles/cm² should be rejected. Diamond wafers of a defect densityless than 300 particles/cm² pass the examination. A diamond substratewith a thin carbon coating of less than 10 nm is permitted for a diamondsubstrate for producing SAW devices. Preferable roughness of the coatingcarbon is less than Ra 10 nm. But less than Ra 20 nm is allowable forthe surface roughness of the carbon coating.

Preferably the diamond film should be wider than 2 inches in diameter(19 cm²) for the standpoint of the economy of wafer processes. However,SAW devices can be fabricated on a diamond wafer less than 19 cm² aslong as the wafer satisfies the other requirements.

The method of fabricating a SAW device is explained by referring to FIG.5 to FIG. 9. A homogenous diamond bulk crystal made by the ultrahighpressure synthesis can be a starting substrate. However, this exampleadopts a diamond/mother complex substrate made by the vapor phasesynthesis method. This example shows a substrate consisting of a silicon(Si) mother wafer and a diamond film deposited on the silicon wafer bythe microwave plasma CVD method. Si single crystal wafer is the mostpopular mother board. Of course, the Si wafer in FIG. 5 can be replacedby a wafer of molybdenum (Mo), nickel (Ni), aluminum nitride (AlN),alumina (Al₂O₃), gallium arsenide (GaAs) or so. A wafer having adiameter more than two inches (19 cm²) is desirable for the viewpoint ofproduction efficiency.

The synthesized diamond film has a rugged surface having a plenty ofconcavities and convexities. Then, the diamond surface is whetted by adiamond-electrodeposited whetstone which is produced byelectrodepositing diamond powder on a turn-table. FIG. 6 shows a dia/Sicomplex wafer polished and flattened by the diamond-electrodepositedwhetstone. The surface roughness should be less than Ra 20 nm.Preferably the surface should be further flattened to a smoothness lessthan Ra 10 nm. The flattened (dia/Si) wafer is now temporarily coated byevaporation with a thin metal, e.g., aluminum film as shown in FIG. 2 orFIG. 3(b) and examined by the laser scanning surface defect detectionapparatus of FIG. 4 for searching the defect density. The test clarifiesthe surface roughness (≦Ra 20 nm), the defect density (≦300 particlescm⁻²) and the carbon coating thickness (≦10 nm) of the object dia/Siwafers. Wafers failing the test should be rejected.

Dia/Si wafers which passed the test are employed as substrates of SAWdevices. The metal thin film is removed from the wafer. A piezoelectricfilm is deposited upon the wafer, for example, by sputtering. FIG. 7shows a zinc oxide (ZnO) film for the piezoelectric film. Otherpiezoelectric materials, e.g., PZT, LiNbO₃ and LiTaO₃ are alsoavailable. A metal film, e.g., Al film is further formed on thepiezoelectric film by sputtering or evaporation. FIG. 8 shows theAl/ZnO/dia/Si wafer. Photolithography produces interdigital transducerson both sides on a SAW chip by masking the metal film with a resist andetching of the metal by an etchant. FIG. 9 shows the interdigitaltransducers formed on the wafer by photolithography. There are varioustypes of interdigital transducers. FIG. 15 denotes an example of aninterdigital transducer which consists of two branches having tworeciprocally facing parallel stripes in a wavelength λ.

EMBODIMENT 1 Defect Density and Electrode Breaking Rate

A SAW device is fabricated by a series of steps of growing a diamondfilm on a mother material substrate, polishing the diamond film,depositing a piezoelectric film, evaporating a metal film on thepiezoelectric film and etching the metal film through a mask. However,the estimation of the diamond should be finished before the formation ofthe piezoelectric film. Thus the electrode breaking rate should bemeasured by forming interdigital transducers directly on the diamondfilm without depositing the piezoelectric film and counting the spots ofbreaking electrode patterns. If the diamond has a poor quality, theinterdigital transducers would break at many spots. The factor affectingthe electrode breaking rate has been sought by forming an interdigitaltransducer directly upon the diamond, measuring the defect density andconsidering the correlation between the breaking rate and the diamondfilm.

1. Growth of Diamond Films

A 35 μm thick diamond film is grown on the silicon wafer of a 1 mmthickness by supplying hydrogen gas containing methane gas of 2 vol% andexciting the mixture gas by the microwave plasma CVD method. 100 samplesare produced by growing 100 diamond films on 100 Si wafers along thesame way. The surface roughnesses of the as-grown diamond wafers are, ingeneral, more than Ra 1 μm.

(Condition of growing diamond films) Substrate silicon wafers: thickness1 mm Microwave power 150 W Material gas methane + hydrogen CH₄:H₂ =2:100 flux 50 sccm Gas pressure 40 mTorr Temperature(substratetemperature) 800° C. Thickness 35 μm

The microwave CVD made dia/Si complex wafers as shown in FIG. 5. The 100samples were polished by a polishing machine having a diamond whetstone.The polishing brought about even dia/Si wafers having good smoothness ofless than Ra 2 nm and few defects of less than 50 particles cm⁻² defectswhich were bigger than 0.5 μm in diameter as shown in FIG. 6. Theaverage thickness of the diamond films was 20 μm. The X-ray diffractionpattern was examined by the X-ray diffraction apparatus. The diffractionpattern revealed an existence of a carbon layer on the diamond film. Thethickness of the carbon layer was less than 10 nm.

2.Deposition of a Piezoelectric Film

Preliminarily 30 nm thick aluminum (Al) films were deposited upon thediamond films of the dia/Si samples by the DC sputtering method. Thealuminum film coating enables the laser scanning surface defectdetection apparatus to examine the surface defects on the diamonds.

(Condition for deposition of aluminum films) Aluminum film thickness 30nm DC sputtering power 1.0 kW Reaction gas argon gas 50 sccm Gaspressure 1.0 Pa (7.6 mTorr) Substrate temperature room temperature

Since the Al films cover the dia/Si samples, the laser scanning surfacedefect detection apparatus can detect defects or dust on the diamondfilms. This apparatus shoots slantingly the Al-coating diamond film withlaser beams, and measures the power of the light scattered from thedefects or dust by the photomultiplier(or photomulti-tube). A defect isan inherent anomaly but dust is a foreign material lying on the diamond.If the diamond is smooth and flat with little defects, the scatteringlight power is weak. If the diamond is rugged with many defects, thescattered light power is strong. The defect density at the object pointis known by the photocurrent. As shown in FIG. 4, since the wafer ismoved in parallel or in spiral, the existence of defects or dust isexamined at all points on the wafer. In the measurement, the biasvoltage of the photomultiplier is 550 V.

The relation between the size of defect and the scattered light power ispreliminarily determined by the steps of coating a wafer with standardLatex granules of a definite diameter, measuring the power of the lightscattered at the granule, replacing the Latex granules with Latexgranules of another diameter, measuring the power scattered by the Latexgranule again and searching the relation between the granule sizes ofLatex and the photocurrent.

Here, the purpose is to form interdigital transducers (IDTs) of a 1 μmline width The Inventors think that defects with a small size less thanhalf of the width have little influence on the electrode formation. Butdefects with a large size more than half of the width decrease the yieldof IDTs. The Inventors considered that the critical size (d) of defectsshould be determined as a half (W/2) of the width (W) of interdigitaltransducers. For the formation of the electrodes of a 1 μm width, it isimportant to know the number of the defects larger than 0.5 μm. Howeverwithout microscope observation, the sizes of individual defects cannotbe measured. Thus the defect was identified by employing Latex standardgranules of a 0.5 μm diameter. A threshold photocurrent was determinedas the current induced by a standard granule of a 0.5 μm diameter. A 0.5μm diameter defect will cause the same increase of the scattering lightpower as the 0.5 μm Latex standard granule. The scattered light power iscalibrated by Latex standard granules, since the size of the standardgranules is uniformly determined. Thus the apparatus can sense surfacedefects which are larger than 0.5 μm in diameter.

Besides the diameter d(≧W/2), the depth or height (h) of a defect isanother factor affecting the electrode breaking rate. Here, the criticaldepth (or height) is determined to be 10 nm for a 1 μm width electrode.The defects which satisfy d≧W/2 and h≧10 nm should be detected andmeasured.

Ten samples were prepared by forming diamond films on ten silicon wafersby a similar method. Ten samples are symbolized by A, B, C, . . . J. Tensamples were examined with regard to the surface roughness and thedefect/dust number. Table 1 shows the surface roughnesses (Ra) anddefect numbers of ten samples. The aluminum films were removed, when thedefect measurement has once been done. The diamond surfaces appeared.

Interdigital transducers were made on the diamond layer of the dia/Siwafers. The electrode pattern breaking rates were measured. The formerthin Al films aimed at measurement of defects. The present Al coating isdirected to making electrodes. Both steps were, by chance, aluminumcoating. But the purposes were different. The formerly coated aluminumwas eliminated. A plurality of interdigital transducers were made byevaporating or sputtering aluminum on the (dia/Si) wafers, making a maskpattern by photolithography, and etching the aluminum layer. A pluralityof pairs of interdigital transducers were produced on each wafer. Thedevices were examined by supplying currents for searching the electrodebreaking. The breaking rate was calculated by dividing the number of theelectrode-breaking devices by the total number of devices. The electrodebreaking rate is also shown in Table 1. The breaking rate is given by anexperimental equation which is called “Murphy's plot”. Murphy's plot isexpressed by Cexp(−DA), where C is a constant, D is the defect densityand A is the area of IDT patterns. This equation means that if the areafor interdigital transducers is wider, the defect density should besmaller for keeping the same yield.

It is a general agreement that if the defect size (W) is larger than ahalf of the minimum pattern width (d), the yield of the IDTs willdecrease. For making electrodes having a width of 1 μm, the defectslarger than 0.5 μm tend to plague the formation of electrodes. Here the“defects” include dust particles and inherent defects. Table 1, thethird column, shows the result of measurement of the number of defectslarger than 0.5 μm for (diamond wafer) samples A to I. Meanderingpatterns with 400 electrode stripes of a 1 μm length×500 μm width wereformed in a spatial period of 1000 μm×500 μm on two inch diameterdiamond wafers. The line width (length) of an individual stripe is 1 μm.The distance between neighboring stripes was 1 μm. Thus, 400 stripeshave a width of about 800 μm. The total area of a wafer is about 19cm²=1900 mm². One spatial period is 1000 μm×500 μm=0.5 mm². About 3800sets of 400 electrode stripes were made on each diamond wafer. Then theelectrodes were examined by supplying currents. Some electrodes turnedout to be broken due to the roughness of the diamond film. The number ofbreaking electrodes was counted for every diamond wafer sample. Thebreaking rates were also listed in Table 1.

TABLE 1 Roughness Ra(nm), Defect density and Breaking rate of aluminumelectrodes of samples A to J of (dia/Si)-substrates BREAKING SAMPLEROUGHNESS DEFECTS DENSITY RATE OF Al NO. Ra (nm) (particles cm⁻²)ELECTRODES(%) A 7.2 80   0% B 8.2 287   0% C 7.6 462  1.2% D 8.3 812 4.8% E 8.9 1431 11.3% F 7.8 2436 33.8% G 9.8 3788 72.3% H 7.6 468991.8% I 8.2 6126  100% J 21.9 Not examined Not made

The surface roughnesses Ra are dispersed between 7.2 nm and 21.9 nm.Sample J had too big surface roughness Ra=21.9 nm which forbade theformation of interdigital transducers. Having the same roughness Ra=7.6nm, sample C and sample H have different breaking rates 1.2% and 91.8%respectively. This means that the surface roughness is not a criticalfactor for determining the breaking rate.

Sample E with Ra=8.9 nm shows a good breaking rate 11.3% which is muchbetter than a breaking rate 100% of sample I of Ra=8.2 nm. The breakingrates are reversed to Ra for samples E and I. Sample H, suffering from ahigh breaking rate of 91.8%, is useless for the substrate of SAWdevices. However, a person who pays attention only to Ra would considerthat sample H must be a good substrate. Ra is not a reliable factor forforeseeing the quality of electrodes formed upon the sample.

Then, the breaking rates are now considered in a different standpoint.Sample B enjoys 0% of pattern breaking rate, since it has a small defectdensity of 287 particles/cm². Sample A exhibits 0% breaking rate due tothe very small defect density of 80 particles/cm². Sample C has still alow defect density of 462 particles cm⁻² which suppresses the breakingrate to 1.2%. Sample D has a defect density of 812 particles cm⁻² whichis about twice as many as sample C. The electrode breaking rate is alsodoubled to 4.8%. Sample F which has defects of 2436 particles cm⁻² isaccompanied by a high breaking rate 33.8%. Sample G with a defectdensity 3788 particles cm⁻² is afflicted with a high breaking rate72.3%. Despite low Ra=7.6 nm, sample H has a very high breaking rate91.8% due to the large defect density of 4689 particles cm⁻². What theseresults tell is that the most important factor of determining thequality of electrodes is not the surface roughness Ra but thedefect/dust density. Namely, the examination by the laser scanningsurface defect detection apparatus gives the most suitable estimationfor the surface states of diamond wafers.

An ideal condition is, of course, 0% of breaking rate. For 0% ofbreaking rate, the defect (and dust) density should be less than 300particles cm⁻², because sample B of 0% has a defect density less than300 particles cm⁻² but sample C of 1.2% has a defect density more than300 particles cm⁻². Thus, 300 particles cm⁻² is the critical valueruling the break of electrode patterns. The roughness must be less than20 nm. The roughness is not a major factor for determining the breakingrate. The roughness less than Ra=10 nm is more suitable for decreasingthe breaking rate.

What affects the break of electrode patterns is not the surfaceroughness but the defect density. There has been no means for estimatingdefects of diamond crystals. The Inventors succeeded in examining thedefect on diamond crystals by coating the object diamond with a thinmetal film and measuring the scattering light in the laser scanningsurface defect detection apparatuses which have been widely employed forestimation of silicon wafers. The new estimation method using the laserscanning surface defect detection apparatus can give a definiterequirement of diamond films or bulks suitable for the substrates of SAWdevices. The preliminary estimation can prophesy that the object diamondcrystals are appropriate for the substrates for making piezoelectricfilms and producing interdigital transducers. SAW devices should be madeonly on the diamond substrates which passed the examination. The sampleswhich fail in the examination should be rejected without makingpiezoelectric films and interdigital transducers (IDT). The estimationcan save much time, energy and resources.

EMBODIMENT 2 Measurement of Propagation Loss on Samples HavingPiezoelectric Films

The previous test tells us that sample A and sample B of less than 300defects/cm² are immune from electrode breaking. However the defectestimation is not sufficient to know the performance of SAW deviceswhich are going to be made by depositing piezoelectric films andelectrodes on the diamond substrates. Then, actual propagation loss wasmeasured on SAW devices made by depositing piezoelectric films on thedia/Si wafers of sample A and sample B which passed the defect test andforming interdigital transducers on the piezoelectric films. No SAWdevices were made on samples C to J which failed in the defect test.

1. Formation of Piezoelectric Films

Piezoelectric ZnO films of a 1050 nm thickness were formed upon thedia/Si substrates of sample A and sample B by an RF sputtering method.

(Condition of formation of ZnO piezoelectric film) Substrates dia/Siwafers : sample A, sample B Target ZnO sintered plate RF power 500 W(13.56 MHz) Reaction gas Ar + O₂ Ar:O₂ = 1:1 gas flow 50 sccm Gaspressure 20.0 Pa (152 mTorr) Substrate temperature 150° C. Speed ofdeposition 5 nm/min Thickness 1050 nm (1.05 μm)

Complex wafers of ZnO/dia/Si were made from samples A and B by the aboveway.

2. Formation of Aluminum Films

80 nm thick aluminum films were deposited upon the ZnO films for makingelectrodes by a DC sputtering method.

(Condition of forming aluminum films) DC sputtering power 1.0 kWReaction gas Ar gas gas flow 50 sccm Gas pressure 1.0 Pa (7.6 mTorr)Substrate temperature room temperature Thickness 80 nm

This step made further complex wafers of Al/ZnO/dia/Si.

3. Production of Interdigital Transducers

Interdigital transducers having the following parameters were formed byeliminating parts of the aluminum layer selectively by photolithography.

(Parameters of interdigital transducers) Line Width of IDTs 0.8 μm(central frequency = 1.75 GHz) Number of stripes 40 pairs double stripes(regular type) Aperture of IDTs 50 × wavelength (wavelength = 8 stripewidths λ = 6.4 μm) Distance between the centers of 50 × wavelengthinput- and output- transducers

FIG. 15 shows the IDT pattern. A unit pattern consists of two sets ofstripe pairs. Two neighboring stripes belong to the same transducer.Four stripes construct one wavelength. The width of a blank is equal tothe width of a stripe. The wavelength is eight times as long as thestripe width (λ=8 d). Thus, SAW devices shown in FIG. 9 have beenfabricated.

Transmission loss and the conversion loss of the SAW devices weremeasured by a spectrum network analyzer (HP8753c). AC power of 1 GHz to2 GHz was applied to one side electrode (electrode 1). The electricpower of the input electrode 1 and the electric power of the output(counterpart) electrode 2, that is, scattering parameters were measured.S11 is a reflection power ratio which is a quotient of the reflectedpower to the electrode 1 to the input power of the electrode 1, when theAC power is supplied to the electrode 1. S22 is a reflection power ratiowhich is a quotient of the reflected power to the electrode 2 to theinput power to the electrode 2, when AC power is supplied to theelectrode 2. S12 is a transmission power ratio which is a quotient ofthe power arrived at the electrode 2 to the input power to the electrode1, when AC power is supplied to the electrode 1. S21 is a transmissionpower ratio which is a quotient of the power arrived at the electrode 1to the input power to the electrode 2, when AC power is supplied to theelectrode 2.

FIG. 10 shows the transmission power ratio (S12) as a function of thefrequency of signals. The abscissa is the frequency (MHz). The ordinateis the transmission power ratio (dB). 1.78 GHz (1780 MHz) gives a peakof the transmission power of −8.2 dB. The frequency corresponds tof=v/λ, where v is the speed of the SAW and λ is the spatial wavelengthof the interdigital transducer which is a predetermined parameter, asshown in FIG. 15. It is a matter of course that the transmission powerratio is the biggest at 1.78 GHz=v/λ. The maximum transmission powerratio is −0.82 dB. The loss 8.2 dB includes all the resistance loss atelectrodes, the bilateral loss and the conversion loss. The realtransmission loss can be calculated by subtracting these losses from thetotal loss 8.2 dB.

The resistance loss is calculated to be 1.0 dB for an electrode stripeof a 80 nm thickness and a 0.8 μm width. The bilateral loss is 6 db,because the power is halved at the input electrode. FIG. 11 shows S11and FIG. 12 shows S22. The conversion loss is known from the loss of theflat regions of S11or S22 except f=1.78 GHz. The loss is about 0.3 dBboth for S11 and for S22. The conversion loss occurs at both the inputand the output electrodes. Then, the conversion loss is 0.6 dB at bothelectrodes. The sum of the resistance loss) the bilateral loss and theconversion loss is 7.6 dB. The total loss for 1.78 GHz is 8.2 dB fromS12 . The propagation loss is only 0.6 dB which is a difference betweenthe total loss 8.2 dB and the sum 7.6 dB of the three losses. 0.6 dB isa small propagation loss.

This is the total propagation loss along the whole path from the inputelectrode to the output electrode. The distance between the centers ofthe electrodes is 50 times as long as the wavelength. The propagationloss per wavelength is 0.012 dB which is calculated by dividing 0.6 dBby 50 λ. The propagation loss 0.012 dB/λ is a very small value for sucha high frequency 1.8 GHz. The low propagation loss is a very conspicuousadvantage of the SAW devices of the present invention. Table 1 showsthat sample A and sample B are free from electrode breaking. Sample Ahas a surface defect density of 80 particles cm⁻² which is far lowerthan the critical value 300 particles cm⁻². The surface roughness ofsample A is Ra 7.2 nm. Sample B has a higher defect density of 287particles cm⁻² which is still lower than 300 particles cm⁻². Sample Bexhibits the roughness of Ra 8.2 nm. Both sample A and sample B areendowed with low insertion loss of about 10 dB.

Other samples C to I have definite electrode breaking rates and lowyields for producing SAW devices. SAW devices have been fabricated onsamples C to I in a similar manner. The electrodes of the SAW devicesare examined by electric property measurement, optical microscopeobservation and electron microscope observation. Some SAW devices aresuffering from electrode breaking. Others are immune from electrodebreaking. The properties of SAW devices without electrode breaking aretested. All the samples of C to I have insertion loss higher than 13 dB.This examination reveals bad crystallographic properties of thepiezoelectric films of samples C to I. The surface defects of diamondfilms worsen the crystallographic properties of the piezoelectric films.The bad crystallography of piezoelectric films raises the propagationloss of the SAW devices made on samples C to I.

This invention examines surface defects of diamond films or bulks bycovering the object diamond surfaces with thin metal films and measuringdefects by the laser scanning surface defect detection apparatus. Thepreliminary examination can foresee the properties of SAW devices whichwill be made on the diamond crystals. SAW devices should be fabricatedonly upon the samples having diamond surface of small defect density,for example, less than 300 particles cm⁻². Here, the definition of adefect includes the critical size which depends upon the period of theelectrodes. Thus, the defect density of the same diamond is differentfor different periods of the electrodes of SAW devices. Low defectdensity diamond films or bulks bring about zero or small enoughelectrode breaking rates when electrodes are made thereon. Lowpropagation loss SAW devices can be produced by depositing piezoelectricfilms on the low defect density diamonds and making interdigitaltransducers on the piezoelectric films.

What we claim is:
 1. A diamond wafer comprising: a bulk diamond singlecrystal or polycrystal substrate synthesized by an ultrahigh pressuresynthesis method or a vapor phase deposition method or a diamondsubstrate consisting of a diamond board or another material board and asingle crystal or polycrystal diamond film synthesized upon the diamondboard or another material board; characterized by surface defect densityof less than 300 particles cm⁻² which is measured by coating the diamondsurface with a thin opaque film, launching laser beams to the coateddiamond surface from a laser scanning surface defect detection apparatusfor letting the laser beams reflect at a surface of the coating film andan interface between the coating film and the diamond surface, andcounting the beams scattered by surface defects or particles on thediamond surface.
 2. A diamond wafer comprising: a bulk diamond singlecrystal or polycrystal substrate synthesized by an ultrahigh pressuresynthesis method or a vapor phase deposition method or a diamondsubstrate consisting of a diamond board or another material board and asingle crystal or polycrystal diamond film synthesized upon the diamondboard or another material board; and a piezoelectric film deposited uponthe diamond substrate or the diamond film; characterized by surfacedefect density of less than 300 particles cm⁻² which is measured bycoating the piezoelectric film surface with a thin opaque film,launching laser beams to the coated piezoelectric film surface from alaser scanning surface defect detection apparatus for letting the laserbeams reflect at a surface of the coating film and an interface betweenthe coating film and the piezoelectric film surface, and counting thebeams scattered by surface defects or particles on the diamond surface.3. A diamond wafer as claimed in claim 1, wherein the diamond substrateis used for making SAW devices or microelectronic devices having wiringpatterns and the surface defects to be counted in are defects havingwidths larger than 50% of the smallest width of wiring patterns andhaving either depths deeper than 10 nm or heights higher than 10 nm. 4.A diamond wafer as claimed in claim 3, wherein roughness of the diamondsurface is less than Ra 20 nm.
 5. A diamond wafer as claimed in claim 4,wherein the diamond substrate has an area which is equal to or widerthan 19 cm².
 6. A diamond wafer as claimed in claim 5, wherein innerstress in the diamond crystal is less than 1.5×10⁸ Pa.
 7. A diamondwafer as claimed in claim 5, wherein the diamond wafer has a distortionheight of less than ±40 μm at a center for a reduced diameter to 50 mm.8. A diamond wafer as claimed in claim 1, wherein the diamond substrateconsists of a diamond board or another material board and a singlecrystal or polycrystal diamond film deposited on the board by a vaporphase synthesis method, the thickness of the board is 0 to 2000 μm andthe thickness of the diamond film is more than 1 μm.
 9. A diamond waferas claimed in claim 1, wherein the thin opaque film coating the diamondsurface for estimation is a metal film or a non-metal film of areflection rate of more than 50%.
 10. A diamond wafer as claimed inclaim 9, wherein the metal film coating the diamond surface is analuminum film deposited by sputtering.